This invention relates generally to optical subassemblies and more particularly to optical subassemblies for passively aligning optical devices to optical fibers.
Optical devices are widely utilized in modern high speed telecommunication networks for reasons including, but not limited to, their very high bandwidth capabilities. Light transmitted by fiber optic cable is, in most instances, produced by a light emitting semiconductor device, such as a vertical cavity surface emitting laser (VCSEL). These devices must be aligned and optically coupled to an end face of the fiber optic cable. The most common alignment process is active alignment, whereby the optical source is aligned to the optical fiber by detecting and maximizing the light coupled into the optical fiber. Active alignment is time consuming and therefore costly. Thus, efforts have been made to produce optical subassemblies that passively align with the optical fibers. The general goals of passively aligned optical sub assemblies are to achieve high coupling efficiency (i.e., low loss of light from the coupling), ease of assembly, and comparatively low cost with respect to actively aligned devices.
Alignment difficulties are introduced by characteristics of both the fiber and the optical device. The core of a typical fiber optic cable is quite small. For example, the core diameter for a single mode fiber is approximately 9 micrometers. Semiconductor lasers typically emit light with elliptical beams in the range of approximately 2-20 micrometers. Difficulties arise in aligning or focusing the laser beam in the fiber. Alignment of the laser and the fiber is critical in order to maximize coupling efficiency, i.e., produce the maximum transmission of light from the laser into the fiber. Misalignment of as little as two micrometers between the optical device and the optical fiber can significantly reduce the optical coupling efficiency.
As stated, the traditional method for coupling an optical fiber to an optoelectronic device utilizes active alignment. For example to couple a VCSEL to an optical fiber by active alignment, the laser is first turned on to emit optical radiation. A coupling end of the optical fiber is then placed near a light emitting surface of the laser to receive optical radiation, and a photodetector is placed at the other end of the fiber to detect the amount of optical radiation that is coupled into the fiber. The position of the coupling end of the fiber is then manipulated manually around the light-emitting surface of the laser until the photodetector at the other end of the fiber detects maximum optical radiation. Optical epoxy is then applied to both the laser and the coupling end of the fiber so as to permanently maintain the optimized coupling. It will be appreciated that the time and labor needed to align the optical device with the optical fiber adds considerable cost to the manufacturing process and limits high volume production of optical components. To avoid the aforementioned problems, a passive alignment approach in which no alignment adjustments are required is generally preferred.
In this regard, some prior art assemblies which employ mechanical positioning approaches aimed at eliminating or reducing the need for active alignment have been developed. One such approach is disclosed in U.S. Pat. No. 5,434,939, issued to Matsuda. As shown in FIG. 1, a VCSEL 10 is formed on a light-emitting chip 12. The light-emitting chip 12 is attached to a monocrystalline silicon (Si) submount 14 which is etched, typically with an anisotropic etchant, to form a guide hole 16. Typically, the light-emitting chip and the submount are self-aligned so that the positions of the VCSEL, and the guide hole, are adjusted. Next, an optical fiber 18 is inserted into the guide hole and fixed with UV cured epoxy 20, which results in the final alignment between the VCSEL and the optical fiber.
Referring to FIG. 2, the guide hole 16 of Matsuda has a much larger outer aperture 22, typically on the order of 870 micrometers than the inner aperture 20, which is typically on the order of the fiber diameter of about 125 micrometers. Thus, the fiber 18 is relatively free to move in all directions within the guide hole. Therefore, the forces normally exerted upon the fiber during the epoxy cure cycle can readily cause misalignment between the fiber and the VCSEL. Thus, there remains room for improvement in the art.
What is needed is a method and apparatus for automatic passive alignment of fiber optic cables or optical waveguides with optoelectronic devices that does not require time consuming and labor intensive manual active alignment, and is less susceptible to misalignment during assembly.
An exemplary embodiment of the optical subassembly of the present invention includes a silicon submount mated to a substrate carrying an optoelectronic device, such as a VCSEL, where the submount passively aligns an optical fiber with the optoelectronic device. The submount includes first and second sides with a midpoint therebetween. A guide hole extends through the first and second sides, where the guide hole includes a first guiding or tapered portion which extends from the first side to about the midpoint of the submount. The tapered portion serves to guide an optical fiber into the hole. The hole also includes a close tolerance or second alignment portion of substantially constant cross section that extends from the midpoint to the second side of the hole. The second portion serves to align the optical fiber with an optical device. Generally, the submount includes on its second side a number of metal deposits for joining the submount, via solder balls, to mating metal deposits formed on a ceramic substrate. The ceramic substrate includes an optoelectronic which is centered with respect to the hole in the submount.
In additional embodiments, the optical subassembly may include a submount having a plurality of guide holes wherein each of the guide holes has a first tapered portion for guiding one of a plurality of optical fibers into a second portion of substantially constant cross section where each of the plurality of optical fibers is aligned with a corresponding optoelectronic device.
The method for fabricating the optical subassembly of the present invention is, in general, as follows. Initially, a silicon nitride layer is grown on both the first and second sides of a double sided polished silicon wafer of predetermined dimensions. A photoresist layer is deposited, or spun as the process is commonly referred to, on the first side of the wafer. The photoresist is then selectively removed to form a void pattern on the silicon nitride layer in the first side of the wafer. The exposed nitride layer is selectively removed with an etchant to form another coextensive void pattern on the exposed silicon in the silicon nitride layer. Next the photoresist layer is removed. Subsequently, the exposed silicon is etched with an anisotropic etchant to form the first tapered portion of the guide hole for the fiber optic cable. At this point, the remaining nitride is removed from both the first and second sides of the silicon wafer and the wafer is flipped over and a new layer of photoresist is spun onto the second side of the wafer.